Molded nanoparticle phosphor for light emitting applications

ABSTRACT

A molded nanoparticle phosphor for light emitting applications is fabricated by converting a suspension of nanoparticles in a matrix material precursor into a molded nanoparticle phosphor. The matrix material can be any material in which the nanoparticles are dispersible and which is moldable. The molded nanoparticle phosphor can be formed from the matrix material precursor/nanoparticle suspension using any molding technique, such as polymerization molding, contact molding, extrusion molding, injection molding, for example. Once molded, the molded nanoparticle phosphor can be coated with a gas barrier material, for example, a polymer, metal oxide, metal nitride or a glass. The barrier-coated molded nanoparticle phosphor can be utilized in a light-emitting device, such as an LED. For example, the phosphor can be incorporated into the packaging of a standard solid state LED and used to down-convert a portion of the emission of the solid state LED emitter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Non-Provisional applicationSer. No. 13/743,414, filed Jan. 17, 2013, which claims the benefit ofU.S. Provisional Application No. 61/588,377, filed Jan. 19, 2012, theentire contents of which are hereby incorporated herein by reference.

BACKGROUND

There has been substantial interest in the preparation andcharacterization of compound semiconductors consisting of particles withdimensions in the order of 2-100 nm, often referred to as quantum dots(QDs) and/or nanoparticles. These studies have focused mainly on thesize-tunable electronic, optical and chemical properties ofnanoparticles. Semiconductor nanoparticles are gaining substantialinterest due to their applicability for commercial applications asdiverse as biological labeling, solar cells, catalysis, biologicalimaging, and light-emitting diodes.

A particularly attractive potential field of application forsemiconductor nanoparticle is in the development of next generationlight-emitting diodes (LEDs). LEDs are becoming increasingly important,in for example, automobile lighting, traffic signals, general lighting,and liquid crystal display (LCD) backlighting and display screens.Nanoparticle-based light-emitting devices have been made by embeddingsemiconductor nanoparticles in an optically clear (or sufficientlytransparent) LED encapsulation medium, typically a silicone or anacrylate, which is then placed on top of a solid-state LED. Thenanoparticles, excited by the primary light of the solid-state LED, emitsecondary light, the color of which is characteristic of the particulartype and size of the nanoparticles. For example, if the primary emissionof the solid-state LED is blue and the characteristic emission of theparticular nanoparticle is red, then the nanoparticles will absorb aportion of the blue light and emit red light. A portion of thesolid-state LED emission is thereby “down-converted” and the deviceprovides light that is a mixture of blue and red.

The use of semiconductor nanoparticles potentially has significantadvantages over the use of the more conventional phosphors. For example,semiconductor nanoparticles provide the ability to tune the emissionwavelength of a LED. However even after the nanoparticles have beenincorporated into the LED encapsulant, oxygen can still migrate throughthe encapsulant to the surfaces of the nanoparticles, which can lead tophoto-oxidation and, as a result, a drop in quantum yield (QY).

In view of the significant potential for the application of quantum dotsacross such a wide range of applications, including but not limited to,quantum dot-based light-emitting devices, there is a strong need todevelop methods to increase the stability of quantum dots so as to makethem brighter, more long-lived and/or less sensitive to various types ofprocessing conditions. There remain significant challenges to thedevelopment of quantum dot-based materials and methods for fabricatingquantum dot-based devices, such as light-emitting devices, on aneconomically viable scale and which would provide sufficiently highlevels of performance to satisfy consumer demand.

SUMMARY OF THE DISCLOSURE

A molded nanoparticle phosphor for light emitting applications isfabricated by converting a suspension of nanoparticles in a matrixmaterial precursor into a molded nanoparticle phosphor comprising amatrix material and the nanoparticles. The matrix material can be anymaterial in which the nanoparticles are dispersible and which ismoldable. For example, the matrix material can be a polymeric material.If the matrix material is a polymeric material, then the matrix materialprecursor can be a formulation of the appropriate monomers. The matrixmaterial precursor may also contain catalysts, crosslinking agents,initiators, and the like.

The molded nanoparticle phosphor can be formed from the matrix materialprecursor/nanoparticle suspension using any molding technique, such aspolymerization molding, contact molding, extrusion molding, injectionmolding, for example. Once molded, the molded nanoparticle phosphor canbe coated with a gas barrier material, for example, a polymer, metaloxide, metal nitride or a glass. The coating can be applied to themolded nanoparticle phosphor by any coating technique, such as atomiclayer deposition, evaporation coating, spray coating, or brush coating.

The barrier-coated molded nanoparticle phosphor can be utilized in alight-emitting device, such as an LED. For example, the phosphor can beincorporated into the packaging of a standard solid state LED and usedto down-convert a portion of the emission of the solid state LEDemitter.

The foregoing summary is not intended to summarize each potentialembodiment or every aspect of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 illustrate prior art nanoparticle-based light emittingdevices.

FIG. 4 illustrates a method of making a nanoparticle-based lightemitting device with a prefabricated nanoparticle disc.

FIG. 5 is a comparison of emission intensity of an exemplary disclosednanoparticle-based light emitting device and a prior artnanoparticle-based light emitting device.

FIG. 6 shows the performance of an exemplary disclosednanoparticle-based light emitting device.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 1 illustrates a prior art nanoparticle-based light-emitting device100 as described in the background. The light-emitting device 100 hassemiconductor nanoparticles 101 in an optically clear (or sufficientlytransparent) LED encapsulation medium 102, typically a silicone or anacrylate, which is then placed on top of a solid-state LED 103. Theencapsulation medium is contained within a package 104. As describedabove, even after the nanoparticles have been incorporated into the LEDencapsulant, oxygen can still migrate through the encapsulant to thesurfaces of the nanoparticles, which can lead to photo-oxidation and, asa result, a drop in quantum yield (QY).

FIG. 2A illustrates a prior art nanoparticle-based light emitting device200 that addresses the problem of photo-oxidation due to oxygen thatmigrates into the encapsulant 202. The nanoparticles 201 areincorporated into microbeads 205, which are suspended in the LEDencapsulant 202. Bead 205 is illustrated in more detail in FIG. 2B.Nanoparticles 201 are incorporated into a primary matrix material 206.The primary matrix material is preferably an optically transparentmedium, i.e., a medium through which light may pass, and which may be,but does not have to be substantially optically clear. The primarymatrix may be a resin, polymer, monolith, glass, sol gel, epoxy,silicone, (meth)acrylate or the like, or may include silica. Examples ofprimary matrix materials include acrylate polymers (e.g.,polymethyl(meth)acrylate, polybutylmethacrylate, polyoctylmethacrylate,alkylcyanoacrylates, polyethyleneglycol dimethacrylate,polyvinylacetate, etc.), epoxides (e.g., EPOTEK 301 A+B Thermal curingepoxy, EPOTEK OG112-4 single pot UV curing epoxy, or EX0135A and BThermal curing epoxy), polyamides, polyimides, polyesters,polycarbonates, polythioethers, polyacrylonitryls, polydienes,polystyrene polybutadiene copolymers (Kratons), pyrelenes,poly-para-xylylene (parylenes), silica, silica-acrylate hybrids,polyetheretherketone (PEEK), polyvinylidene fluoride (PVDF), polydivinylbenzene, polyethylene, polypropylene, polyethylene terephthalate (PET),polyisobutylene (butyl rubber), polyisoprene, and cellulose derivatives(methyl cellulose, ethyl cellulose, hydroxypropylmethyl cellulose,hydroxypropylmethylcellulose phthalate, nitrocellulose), andcombinations thereof.

Microbead 205 may also include a coating 207 to prevent the passage ordiffusion of oxygen, moisture, or free radicals through the primarymatrix material. The coating may be an inorganic material, such as adielectric, a metal oxide, a metal nitride, or silica. Alternatively,the coating may be another material, such as a polymer material.

Referring again to FIG. 2A, a drawback associated with microbeads 205 isthat they scatter, reflect, and refract light. These optical effectslead to loss of overall performance (brightness) of the light-emittingdevice.

FIG. 3 illustrates a hermetically sealed nanoparticle-basedlight-emitting device 300 that includes a gas barrier film 308 sealed toLED package 304 to prevent migration of deleterious species such asoxygen, moisture, and radicals into encapsulant 302. However, ahermetically sealed device 300 is difficult to manufacture, particularlyon a commercial scale, because materials suitable as a gas barrier 308(e.g., ceramics) are expensive and difficult to work with. It can bedifficult to achieve an impermeable seal between gas barrier 308 and LEDpackage 304, and as a result, deleterious species may still diffuse intothe device at interface 309. Moreover, it has recently been observedthat sealed packaging can cause the development of micro-climate effectsthat deteriorate the performance of the LED wiring and emissive chip. Sowhile it is desirable to prevent oxygen and the like from migrating intothe LED encapsulant material, it is surprisingly also desirable that theLED packaging itself be allowed to “breath.” An object of the presentdisclosure is to provide a nanoparticle-based light emitting device thatfulfills these two seemingly contradictory goals.

FIG. 4 illustrates a method of preparing a nanoparticle-based lightemitting device that overcomes the problems described above. Asuspension 401 of nanoparticles in a matrix material precursor istransferred to a mold 402. Once in the mold, the matrix materialprecursor is converted to the matrix material to yield a moldednanoparticle phosphor 403. Note that molded nanoparticle phosphor isschematically represented as being square-shaped in FIG. 4, but it willbe appreciated that the actual shape of molded nanoparticle phosphor 403will be determined by the shape of mold 402. The disclosure is notlimited to any particular shape. Note also that FIGS. 1-4 are not toscale. Exemplary matrix materials include resins, polymers, sol gels,epoxies, silicones, (meth)acrylates or the like, or may include silica.Examples of matrix materials include acrylate polymers (e.g.,polymethyl(meth)acrylate, polybutylmethacrylate, polyoctylmethacrylate,alkylcyanoacrylates, polyethyleneglycol dimethacrylate,polyvinylacetate, etc.), epoxides (e.g., EPOTEK 301 A+B Thermal curingepoxy, EPOTEK OG112-4 single pot UV curing epoxy, or EX0135A and BThermal curing epoxy), polyamides, polyimides, polyesters,polycarbonates, polythioethers, polyacrylonitryls, polydienes,polystyrene polybutadiene copolymers (Kratons), pyrelenes,poly-para-xylylene (parylenes), silica, silica-acrylate hybrids,polyetheretherketone (PEEK), polyvinylidene fluoride (PVDF), polydivinylbenzene, polyethylene, polypropylene, polyethylene terephthalate (PET),polyisobutylene (butyl rubber), polyisoprene, and cellulose derivatives(methyl cellulose, ethyl cellulose, hydroxypropylmethyl cellulose,hydroxypropylmethylcellulose phthalate, nitrocellulose), andcombinations thereof.

Matrix material precursors may be any precursor formulation in which thenanoparticles can be suspended or dissolved and which can be convertedto the matrix material. For example, if the matrix material is apolymer, then the matrix material precursor may be a formulation of thecorresponding monomer and any additional species, such asphotoinitiators, catalysts, and/or crosslinking agents for convertingthe matrix material precursor to the matrix material. According to oneembodiment, in which the matrix material is an acrylate polymer, thematrix material precursor can be a formulation of the appropriatemethacrylate monomer, a photoinitiator, and a crosslinking agent. Thematrix material precursor can be converted to the matrix material by anymethod known in the art, including by not limited to, photoinitiatedpolymerization.

Mold 402 can be any mold having the shape to produce the desired shapeof molded nanoparticle phosphor 403. According to one embodiment, mold402 is itself LED packaging that is substantially identical to the LEDpackaging to be used in the final nanoparticle light-emitting device.The disclosure is not limited to any particular method of forming moldednanoparticle phosphor 403. Any known molding or casting technique can beused, for example, contact molding, casting, extrusion or injectionmolding.

Molded nanoparticle phosphor 403 can be coated with a gas barriermaterial to yield coated molded nanoparticle phosphor 404. The coatingis preferably a barrier to the passage of oxygen or any type ofoxidizing agent through the primary matrix material. The coating may bea barrier to the passage of free radical species through the primarymatrix material, and/or is preferably a moisture barrier. It will beappreciated that the gas barrier material may not completely prevent thepassage of gas and/or moisture.

The coating may provide a layer of coating material of any desirablethickness. For example, the surface layer coating may be around 1 to 10nm thick, up to around 400 to 500 nm thick, or more. The coating caninclude an inorganic material, such as a dielectric (insulator), a metaloxide, a metal nitride or a silica-based material (e.g., a glass).

Preferred metal oxides are selected from the group consisting of Al₂O₃,B₂O₃, Co₂O₃, Cr₂O₃, CuO, Fe₂O₃, Ga₂O₃, HfO₂, In₂O₃, MgO, Nb₂O₅, NiO,SiO₂, SnO₂, Ta₂O₅, TiO₂, ZrO₂, Sc₂O₃, Y₂O₃, GeO₂, La₂O₃, CeO₂, PrO_(x)(x=appropriate integer), Nd₂O₃, Sm₂O₃, EuO_(y) (y=appropriate integer),Gd₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, Lu₂O₃, SrTiO₃, BaTiO₃, PbTiO₃,PbZrO₃, Bi_(m)Ti_(n)O (m=appropriate integer; n=appropriate integer),Bi_(a)Si_(b)O (a=appropriate integer; b=appropriate integer), SrTa₂O₆,SrBi₂Ta₂O₉, YScO₃, LaAlO₃, NdAlO₃, GdScO₃, LaScO₃, LaLuO₃, Er₃Ga₅O₁₃.

Preferred metal nitrides may be selected from the group consisting ofBN, AlN, GaN, InN, Zr₃N₄, Cu₂N, Hf₃N₄, SiNe (c=appropriate integer),TiN, Ta₃N₅, Ti—Si—N, Ti—Al—N, TaN, NbN, MoN, WN_(d) (d=appropriateinteger), and WNeCf (e=appropriate integer; f=appropriate integer).

The inorganic coating may include silica in any appropriate crystallineform.

The coating may incorporate an inorganic material in combination with anorganic or polymeric material. By way of example, in a preferredembodiment, the coating is an inorganic/polymer hybrid, such as asilica-acrylate hybrid material.

In a second preferred embodiment, the coating includes a polymericmaterial, which may be a saturated or unsaturated hydrocarbon polymer,or may incorporate one or more heteroatoms (e.g., O, S, N, halo) orheteroatom-containing functional groups (e.g., carbonyl, cyano, ether,epoxide, amide and the like).

Examples of preferred polymeric coating materials include acrylatepolymers (e.g., polymethyl(meth)acrylate, polybutylmethacrylate,polyoctylmethacrylate, alkylcyanoacrylates, polyethyleneglycoldimethacrylate, polyvinylacetate, etc.), epoxides (e.g., EPOTEK 301 A+B

Thermal curing epoxy, EPOTEK OG112-4 single pot UV curing epoxy, orEX0135A and B Thermal curing epoxy), polyamides, polyimides, polyesters,polycarbonates, polythioethers, polyacrylonitryls, polydienes,polystyrene polybutadiene copolymers (Kratons), pyrelenes,polypara-xylylene (parylenes), polyetheretherketone (PEEK),polyvinylidene fluoride (PVDF), polydivinyl benzene, polyethylene,polypropylene, polyethylene terephthalate (PET), polyisobutylene (butylrubber), polyisoprene, and cellulose derivatives (methyl cellulose,ethyl cellulose, hydroxypropylmethyl cellulose,hydroxypropylmethylcellulose phthalate, nitrocellulose), andcombinations thereof.

Coating 405 can be applied to molded nanoparticle phosphor 403 by anycoating method known in the art and in related arts, such as thepharmaceutical arts, wherein coating are commonly applied to tablets andthe like. Examples of coating methods include atomic layer deposition(ALD). Other methods include spray coating, evaporative and brushcoating.

Coated molded nanoparticle phosphor 404 is inserted into LED packaging406, which can be filled with an LED encapsulant, such as a silicone orepoxy, and the fabrication of nanoparticle-based light emitting device407 can be completed according to typical practice in the LED industry.

The instant disclosure is not limited to any particular type ofluminescent nanoparticle. In preferred embodiments, the nanoparticle isa semiconductor material. The semiconductor material may incorporateions from any one or more of groups 2 to 16 of the periodic table, andmay include binary, ternary and quaternary materials, that is, materialsincorporating two, three or four different ions respectively. By way ofexample, the nanoparticle may incorporate a semiconductor material, suchas, but not limited to, CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, InP, InAs,InSb, AlP, AlS, AlAs, AlSb, GaN, GaP, GaAs, GaSb, PbS, PbSe, Si, Ge andcombinations thereof. According to various embodiments, nanoparticlesmay have diameters of less than around 100 nm, less than around 50 nm,less than around 20 nm, less than around 15 nm and/or may be in therange of around 2 to 10 nm in diameter.

Nanoparticles that include a single semiconductor material, e.g., CdS,CdSe, ZnS, ZnSe, InP, GaN, etc. may have relatively low quantumefficiencies because of non-radiative electron-hole recombination thatoccurs at defects and dangling bonds at the surface of thenanoparticles. In order to at least partially address these issues, thenanoparticle cores may be at least partially coated with one or morelayers (also referred to herein as “shells”) of a material differentthan that of the core, for example a different semiconductor materialthan that of the “core.” The material included in the, or each, shellmay incorporate ions from any one or more of groups 2 to 16 of theperiodic table. When a nanoparticle has two or more shells, each shellmay be formed of a different material. In an exemplary core/shellmaterial, the core is formed from one of the materials specified aboveand the shell includes a semiconductor material of larger band-gapenergy and similar lattice dimensions as the core material. Exemplaryshell materials include, but are not limited to, ZnS, ZnO, MgS, MgSe,MgTe and GaN. An exemplary multi-shell nanoparticle is InP/ZnS/ZnO. Theconfinement of charge carriers within the core and away from surfacestates provides nanoparticles of greater stability and higher quantumyield.

While the disclosed methods are not limited to any particularnanoparticle material, nanoparticles comprising materials that do notcontain cadmium are particularly favored because of increasing concernover potential toxic and environmental effects associated with cadmium.Examples of cadmium free nanoparticles include nanoparticles comprisingsemiconductor materials, e.g., ZnS, ZnSe, ZnTe, InP, InAs, InSb, AlP,AlS, AlAs, AlSb, GaN, GaP, GaAs, GaSb, PbS, PbSe, Si, Ge, andparticularly, nanoparticles comprising cores of one of these materialsand one or more shells of another of these materials.

EXAMPLE 1

Molded nanoparticle phosphors of the size of a standard 20 mw LEDpackage were prepared by using an actual LED package as a mold. Asolution of CFQD in toluene (for example 20 mg inorganic) is dried undervacuum to leave a QDs residue. To the residue laurylmethacrylate (1.85ml, 6.6 mmol) is added to a solution of the photoinitiator (Irgacure819, 9 mg) dissolved in the crosslinker trimethylolpropanetrimethacrylate (1.06 ml, 3.3 mmol). An aliquot of the monomer mixture(1.5-3 ul) is used to fill the cup of an LED. The filled LED is thenirradiated (Hamamatsu UV-LED lamp LC-L2, 365 nm, 7500 mW/cm², 3minutes). The solidified molded nanoparticle phosphor is then removedfrom the LED by simple tapping and then processed for coating with gasbarrier film (using for examples coating methods such as Atomic LayerDeposition-ALD and or high barrier materials like PVOH). The coated discis then inserted into a new LED package that is filled with a properencapsulating resin.

FIG. 5 shows an emission spectrum produced by a cadmium free quantum dotnanoparticle-based (CFQD) (InP/ZnS) light emitting device using a moldednanoparticle phosphor (curve a), compared to a device usingnanoparticles suspended in microbeads (curve b) of the same matrixmaterial. The emission luminescence peak at about 630 nm issubstantially higher with the molded nanoparticle phosphor device. Theluminescence intensity is attenuated in the microbead-based devicelikely due to scattering by the multiple microbeads, as discussed above.

FIG. 6 shows the efficacy (curve a, the total brightness of the LEDbased on human eye sensitivity), percent photoluminescence intensity ofthe QD peak alone (curve b), QD/LED intensity (curve c, the ratiobetween the QD peak alone and the blue chip peak), and LED intensity(curve d, the blue chip peak alone) a nanoparticle light emitting deviceusing a molded nanoparticle phosphor, as disclosed herein.

The foregoing description of preferred and other embodiments is notintended to limit or restrict the scope or applicability of theinventive concepts conceived of by the Applicants. It will beappreciated with the benefit of the present disclosure that featuresdescribed above in accordance with any embodiment or aspect of thedisclosed subject matter can be utilized, either alone or incombination, with any other described feature, in any other embodimentor aspect of the disclosed subject matter.

What is claimed is:
 1. A light emitting device, the device comprising: alight emitting diode (LED) package, the LED package comprising: ahousing; and an LED located within the housing; and a phosphorcomposition disposed within the housing and in optical communicationwith the LED, the phosphor composition comprising: a molded matrixmaterial; a plurality of nanoparticles suspended within the moldedmatrix material; and a gas barrier material coated upon every surface ofthe molded matrix material.
 2. The light emitting device of claim 1,wherein the matrix material is a resin, a polymer or a sol gel.
 3. Thelight emitting device of claim 2, wherein the resin is an acrylateresin.
 4. The light emitting device of claim 3, wherein the acrylateresin is formed from laurylmethacrylate mononer, a photoinitiator and acrosslinker.
 5. The light emitting device of claim 1, wherein the matrixmaterial is an epoxy, silicone, or acrylate.
 6. The light emittingdevice of claim 1, wherein the gas barrier material is a polymer, ametal oxide, a metal nitride or a glass.
 7. The light emitting device ofclaim 1, wherein the gas barrier material is an epoxy, a silicone, anacrylate, or a silica-acrylate hybrid material.
 8. The light emittingdevice of claim 1, further comprising an LED encapsulation medium. 9.The light emitting device of claim 8, wherein the LED encapsulationmedium is a silicone or an epoxy.
 10. The light emitting device of claim1, wherein each of the nanoparticles comprise a semiconductor material.11. The light emitting device of claim 1, wherein each of thenanoparticles comprise a core and shell at least partially coating thecore, the core having a first semiconductor material and the shellhaving a second semiconductor material.
 12. The light emitting device ofclaim 11, wherein the first semiconductor material is any one of CdS,CdSe, CdTe, ZnS, ZnSe, ZnTe, InP, InAs, InSb, AlP, AlS, AlAs, AlSb, GaN,GaP, GaAs, GaSb, PbS, PbSe, Si, Ge, and any combination thereof.
 13. Thelight emitting device of claim 11, wherein the second semiconductormaterial is any one of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, InP, InAs,InSb, AlP, AlS, AlAs, AlSb, GaN, GaP, GaAs, GaSb, PbS, Pb Se, Si, Ge,and any combination thereof, and wherein the first semiconductormaterial and the second semiconductor mater are different.
 14. The lightemitting device of claim 11, wherein the second semiconductor materialis any one of ZnS, ZnO, MgS, MgSe, MgTe and GaN.
 15. The light emittingdevice of claim 11, wherein the core is InP and the shell is ZnS. 16.The light emitting device of claim 1, wherein the barrier layer has athickness between about 1 nm and about 500 nm.
 17. A method of making alight-emitting device, the method comprising: providing a light emittingdiode (LED) package, the LED package comprising: a housing; and an LEDlocated within the housing; disposing a phosphor composition within thehousing such that the phosphor composition is in optical communicationwith the LED, the phosphor composition comprising: a molded matrixmaterial; a plurality of nanoparticles suspended within the moldedmatrix material; and a gas barrier material coated upon every surface ofthe molded matrix material.
 18. The method of claim 17, furthercomprising encapsulating the LED with an encapsulating medium.
 19. Themethod of claim 18, further comprising encapsulating the phosphorcomposition with the encapsulating medium.
 20. The method of claim 18,wherein the LED encapsulation medium is a silicone or an epoxy.